A vast number of prior art logic circuits employ optical devices to perform optical logic functions and operations. However, many of these circuits involve converting logic level signals between the optical domain and another domain such as the electrical domain. This optical conversion process limits the bandwidth of the optical signals, requires additional processing time, and commonly requires additional circuitry. Generally, the output signal of a nonlinear optical device is a nonlinear gain function of an input signal applied to the device where either the input or the output signal is in the optical domain. In the more specific case of an optically nonlinear optical device, the input and the output signals are both in the optical domain. Consequently, optically nonlinear optical devices can regenerate optical signals and perform optical logic functions such as the optical logic NOR, OR, NAND, and the like. However, optically nonlinear optical devices that employ an optical conversion process still have the aforementioned problems.
With the use of parallel processing techniques, it is often desirable to connect in a parallel manner the optical output of each optical logic element in one array to the optical input of each optical logic element in another array. As a result, the number of individual physical connections using, for example, optical fibers between the two arrays can be enormous with the total equaling the mathematical product of the number of elements in one array times the number of elements in the other array. Depending on the physical size of the elements as well as the interconnections, space considerations can rapidly become a factor limiting the number of connections between two arrays. This is just one reason why optical parallel processing techniques have had such limited acceptance and use.
Another prior art approach for interconnecting optical logic elements uses a computer-generated transmission hologram. Generally, a hologram consists of any material for storing the optical wavefront from an object that is encoded in an optical fringe pattern for subsequent recreation of the wavefront. One familiar example of a hologram for creating artistic visual effects is a photographic plate that has been exposed to the coherent light from a three-dimensional object and a reference beam interfering in the plate. After the photographic plate is developed, the reference beam is again passed through the developed photographic plate to recreate a three-dimensional image of the object.
One example of an optical sequential logic system utilizing computer-generated transmission holograms for optically interconnecting the optical logic elements of the system is described by A. A. Sawchuk et al. in Technical Report No. 1100 entitled "Nonlinear Real-Time Optical Signal Processing", University of Southern California Image Processing Institute, Los Angeles, Calif., 1983. The optical logic system includes an array of computer-generated Fourier transmission holograms for optically interconnecting a similar array of liquid crystal light valves. The light valves are optically nonlinear optical devices and are operated to regenerate optical signals and to perform an optical logic NOR function. However, one disadvantage of the liquid crystal light valve is that the optical input control signals are received on one surface of the device and that the optical output signals are emitted from another surface usually on the other side of the device. Thus, the transmission holograms and a complicated arrangement of precisely positioned lenses and mirrors must direct the optical output signals from the rear surface of the light valve array 360 degrees onto the front surface of the light valve array. The long distance that optical output signals must travel from the rear surface of a light valve before being reflected as an intput control signal onto the front surface of at least one other light valve, severely limits the operating speed of any optical system using this transmission hologram interconnection arrangement. Another problem with this interconnection arrangement is the mechanical precision required in aligning the transmission holograms and the light valves with the mirrors and the lenses. A slight vibration can misalign the entire system.
Another disadvantage of the liquid crystal light valve is its relatively slow switching speed. Faster speed nonlinear Fabry-Perot Interferometers are possible substitutes for the liquid crystal light valves. However, with faster switching speeds, proportional amounts of additional power are required to operate the device.
Another problem with Fourier transmission holograms is the significant power loss of an optical signal as it passes through the hologram. Each optical signal passing through a Fourier transmission hologram forms two images of which only one is used to interconnect the light valves and has at most half the optical power of the incident signal. Furthermore, transmission holograms operate only with coherent light which may result in optical interference at the input of a optical logic element due to constructive and destructive interference between several input signals.